Copy method and apparatus



Aug. 7, 1962 Filed June 22, 1959 R. B. RUSSELL COPY METHOD AND APPARATUS 3 Sheets-Sheet 1 INVENTOR.

ROBERT B. RUSSELL BY gain, s ATTORNEYS 1962 R. B. RUSSELL 3,048,695

COPY METHOD AND APPARATUS Filed June 22, 1959 3 Sheets-Sheet 2 INVENTOR.

ROBERT B. RUSSELL BY 7 0117,, W *1- M ATTOR N EYS Aug. 7, 1962 Filed June 22, 1959 R. B. RUSSELL COPY METHOD AND APPARATUS 3 Sheets-Sheet 3 17 I D WAX TRANSFER SHEET U l 2 COPY SHEET j ORIGINAL FlG.4

WAX MELTING POINT 7: (I L11 0.. 2 LLI FIG.5

INVENTOR.

ROBERT B. RUSSELL BY mmm M ATTORNEY 3,048,695 COPY METHOD AND APPARATUS Robert B. Russell, Newton, Mass, assignor to S. D. Warren Company, Boston, Mass, a corporation of Massachusetts Filed June 22, 1959, Ser. No. 821,943 7 Claims. (Cl. 250-65) This invention relates to copying methods and apparatus, and more particularly to a process and apparatus for producing copy from black or essentially radiant energy absorbent letters on paper or the like.

Industry has long sought a simple, inexpensive and quick way to make permanent copies of printed letters or the like on paper without the need for chemical development or other treatment after a single exposure of the copy and the original. It is a general object of my invention to provide the same.

Other objects and the features of my invention will be understood more readily if the following underlying principles are first set forth.

My invention stems initially from the discovery that, when a piece of paper carrying black letters thereon is placed in contact with another piece of paper placed over the letters, and when this combination is subjected to radiation (i.e. heat-producing light rays) under conditions in which the radiant energy passes through the copy sheet and heats the surface of the black letters more than the surrounding areas of white paper, it is possible to make a substantial portion of the surface heat of the letters pass back up through the copy in the form of radiant rather than conducted heat. Hitherto it has been thought that the heat emanating from such letters is transferred back- Wardly through or into the copy only by or predominantly by conduction, and as long as the industry has been under the influence of such a concept, it has appeared impossible to the industry to make good copy by heat transfer through poorly conducting papers or across the highly insulative air gap interfaces between superimposed layers of paper.

A second fact of basic importance in understanding my invention relates to the degree of absorbency of and/or transparency to radiant energy by various materials at various wave lengths of radiation. Paper is relatively transparent to radiation having a wave length of 1-3 microns, but highly absorbent to radiation longer than 5 microns. According to Chemical Engineers Handbook (1941), paper is very close to a perfect black body (.924 to .944) when exposed to black body radiation at a temperature of 66 F. Since lamp black at 68 F. is rated in the same range (i.e. .927 to .967), paper and lamp black may be regarded as equally absorbent of the type of radiation a black body emits at low temperatures. Exactly what this radiant energy comprises in terms of wave length distribution is not stated, but it is assumed to be predominantly in the long wave length range largely exceeding 5 microns. As the temperature of the black body radiation rises, the concentration of radiation shifts toward the shorter wave lengths and, in the range of 1-3 microns, a vast difference between the radiant energy absorption of white paper and carbon black appears. White paper still absorbs such radiation but to a much less degree than carbon black. These facts are important in the process of my invention because they permit me to transmit radiant energy at approximately 1.2 microns wave length through two layers of paper and selectively heat up black letters (containing carbon) on an original. Then, once such black letters are superficially heated, they re-radiate energy back into and through the sheets through which the exciting energy has just passed. Some of this re-radiated energy is in the form of long wave length radiation which is readily or completely absorbed by paper, but a portion of 3,948,695 Patented Aug. 7, 1962 it is also of a shorter wave length which penetrates paper to some extent While being absorbed therein.

The foregoing basic facts are important in the practise of my invention because they lead to the selection of materials and process conditions not hitherto thought to be important, but which, based on the results I have attained, produce a far superior, permanent copy in less time and with less inconvenience than any previously known economically equivalent method.

In the practise of my invention, in a preferred embodirnent, I place a copy sheet of conventional onionskin thickness over the original, and then place a dyed 'wax transfer sheet over the copy. These combined sheets are then subjected to a short blast of radiation containing in its spectrum a substantial percentage of relatively short wave length penetrating light rays. This light passes through the transfer and copy sheets and impinges on the surface of the original where it is selectively absorbed by the black areas of the letters to a greater extent than the white background areas of the original. The heated black areas then re-radiate heat back into the copy and transfer sheet in the form of longer wave length radiation which is absorbed in the copy and transfer sheet and selectively heat them in the areas of the letters, causing the dyed wax to melt and transfer to the copy sheet.

The resulting cop-y has excellent definition when the refinements of my invention are practised. Among its special advantages is the fact that the copy is permanent. The letters on the copy comprise a layer of dyed wax, and as such they are as permanent as most inks and far more permanent than pencil or soft carbon copy. The cost of such copy is extremely low involving no more than the cost of a single light weight sheet, plus the cost of a wax coated sheet containing a special dye. In practise, the cost is substantially less than one-half the cost per copy of the conventional process sold by Minnesota Mining and Manufacturing Co. under the name Thermofax which I understand is the least expensive comparable dry process now on the market. As for speed, my process can copy a conventional letter-sized sheet (8% x 11") in less than two seconds. As regards appearance, the letters on the copy produced by my method are shiny and slightly raised. This gives them a striking appearance not hitherto available from known comparable copying methods. Another unusual feature of my invention relates to copying halftone prints. My process can reproduce these with more definition than many of the more expensive techniques.

In the practise of my invention a number of precautions should be taken if optimum copy is to be produced. For instance, care must be taken to avoid heating up the wax transfer and copy sheets by the applied radiation any more than necessary. This is done first, by selecting a light source rich in radiation of a wave length which more readily penetrates wax and paper. It is likewise preferable to permit the light to pass a substantial air gap prior to reaching the wax transfer sheet. This tends to eliminate some of the upper portion of the spectrum and helps render the radiation more uni-directional. Of course, a wax and dye combination which is especially transparent to the applied radiation should be selected. The thickness of the wax transfer and copy sheets is likewise an important consideration. They should not be so thick that they absorb too much of the applied radiation as it passes through the sheets or so thick that they prevent the re-radiation of the heat from the letters back through the copy to the wax transfer sheet surface. Another precaution is to direct the light as nearly normal to the plane of the sheets as possible in order to maintain the path of the light through the transfer and copy sheets as short as possible. Another precaution is to employ papers which avoid scattering the radiation any more than necessary.

While the above-stated precautions are important, it

of the transfer sheet and melt the wax. If these conditions are met, then the system is not dependent upon conduction through the copy or across the air gap interfaces between the sheets, and good copy is possible. It is a feature of my invention that I can employ papers thicker than conventional onionskin copy papers for my copy paper, and by adjusting the other variables, easily achieve adequate selective return radiation from the letters to melt the wax in the areas of the letters. Much thicker copy sheets may even be employed by proper adjustment of the above variables.

Another important consideration in the practise of my invention which merits careful analysis is the intensity of the applied radiation which reaches the surface of the original. The more intense this light is, the greater will be the contrast between the temperature of the letters and the temperature of the background on the surface of the original. This fact is of particular importance in my invention because of the need for generating a sufficiently intense and penetrating counter-radiation of heat from the letters to penetrate back through a copy sheet of substantial thickness and melt the surface of the wax transfer sheet beyond. Moreover, with my invention accumulating or building up heat in the body of the letters is to be avoided. I find that such accumulated heat tends to distribute itself laterally and give the copy letters a fuzzy and indistinct appearance, and also to cause unwanted background to appear on the copy. I believe this is due to the tendency of such heat to follow the path of the fibers in the paper and to resist jumping across the air gap at the interfaces between the various sheets. By increasing the intensity of the applied light, I find I can almost instantaneously cause the temperature of the surface of the letters to reach a point at which the counter radiation from the letters is sufficiently rapid to melt the wax surface of the transfer mat, and that the time required for this can be so short that the accumulation of heat in the letters is not excessive. This virtually avoids the harmful effects of lateral heat conduction. This feature of my invention has a particular bearing in the reproduction of letters of different sizes and/or ink thicknesses. Since larger and thicker letters accumulate more heat than smaller and thinner ones, the heat accumulated therein during exposure to the light spreads further from the boundary line of the larger and thicker letters after exposure. This leads to uneven copying in a system which depends on conduction, but can be avoided or minimized in the system of my invention.

Another feature of my invention relates to the duration of contact between the Wax transfer sheet and the copy sheet. It will be understood that my invention may be practised even though an excess of heat is actually stored or accumulated in the letters during exposure. Under such conditions the sharpness of the letters can be preserved by separating the wax transfer sheet from the copy sheet before the stored or accumulated heat has had a chance to spread or pass through the copy in the unwanted areas. This is a feature of both the process and the apparatus of my invention.

Still another feature of my invention relates to the closeness of contact between the original, copy sheet and Wax transfer sheet, and the point at which in the process such closeness of contact is important. Since the applied radiation is relatively penertating, closeness of contact is not particularly important in connection with selectively heating the letters. However, since the counter-radiation from the original letters spreads in all directions at once and since the counter radiation starts almost as soon as the applied heat starts, it is desirable to keep the copy and wax transfer sheets relatively close during exposure. However, extremely close contact at such times is not critical and a feature of my invention illustrating this is that deeply imprinted black letters, which never touch the copy at all, result in just as good coppy by my method as black letters which are flush with the surface of the original. Extremely close contact, however, is important immediately after exposure to the light. Once the Wax is melted, or softened enough to transfer, a complete and uniform contact between the transfer and copy sheets is highly desirable, and it is a feature of my invention that I provide the same.

Further objects and features of my invention will best be understood and appreciated from the following detailed description of a preferred embodiment thereof, selected for purposes of illustration, and shown in the accompanying drawings, in which:

FIG. 1 is a cross sectional view in side elevation taken through the center of the apparatus of my invention;

FIG. 2 is a partially exploded view in perspective illustrating essentially the same elements shown in FIG. 1;

FIG. 3 is an enlarged view in cross section and side elevation of the light, reflector and printing roll;

FIG. 4 is an enlarged vie-w of the original, copy sheet and wax transfer sheet employed in my invention; and

FIG. 5 is a graph illustrating certain aspects of the operation of my invention.

The preferred embodiment of the apparatus of my invention herein shown includes in its general organization a base 10 supporting side walls 12, which serve in turn to support in suitable bearings a series of rolls for feeding the elements of the copying combination through the machine. These rolls include a wax transfer sheet supply roll 14 carrying a supply of wax transfer sheet 15, a copy sheet supply roll 16 carrying a supply of copy sheet 17, a wax transfer sheet retarding roll 18 operatively engaged by a pressure roll 20 to grip the wax transfer sheet 15 against the retarding roll 18, a printing roll 22, a light source indicated generally at 24, an idler roll 26, a wax transfer sheet drive roll 28 driven by a motor 30, and a pressure roll 32 operatively engaging drive roll 28 to grip and drive the wax transfer sheet 15. The rotation of the retarding roll 18 is limited by a friction brake arrangement indicated generally at 34.

With the foregoing elements arranged in series, the wax paper is threaded through the machine, around the retarding roll 18, under the printing roll 22, between it and the light source 24, over the idler roll 26, and thence between the drive roll 28 and its pressure roll 32. From this point the wax transfer sheet 15 issues to waste through an exit chute 36 in the rear of the machine. The copy sheet 17 is fed forwardly from the roll 16 on top of the wax transfer sheet 15, between it and the printing roll 22, and thence outwardly towards the front of the machine to an exit opening 38. A small take-off roll 40 is employed to insure separation of the copy sheet 17 from the wax sheet 15 immediately after the two have passed adjacent to the light source 24. A cut-off knife 42 is provided adjacent to the opening 38 for the purpose of cutting the copy sheet 17 to appropriate length when a copy has been made.

An original to be copied is indicated at 44 and is introduced through a slot 46 in position over the copy sheet 17 from which point it is likewise fed under the printing roll 22 and issues along with the copy sheet from the opening 38.

Further details of the apparatus of my invention will best be understood if considered along with a discussion of the operation of the process thereof. in operation, the wax transfer sheet 15 is placed with its coated side adjacent to the copy sheet 1'7, and the original 44 is likewise placed with the letters to be copied thereon facing the other side of the copy sheet 17. As these three pass under the printing roll 2-2 and come into the area of application of the radiation from the light source 24, the radiation penetrates down through the transfer sheet 15 and copy sheet 17 to the surface of the original 44 where the light selectively heats the surface of the letters on the original to a greater extent than it heats the background areas on the surface of the original. At this point the letters reradiate heat backwardly through the copy sheet 17 to the wax sheet 15, melting the same and thereby preparing the same for transfer to the copy sheet in the areas of the letters.

In order to produce a light which will penetrate the wax transfer and copy sheets without heating them up excessively, I employ an infra-red lampsold by the General Electric Company under the designation l000T3, which lamp is adapted to operate at 220 volts for normal operation. I find, however, that at such a voltage this lamp emits a spectrum which is overly absorbed by the wax transfer and copy sheets. Preferably I operate the lamp at 440 volts, and for some uses it may even be preferable to operate the lamp at higher voltages. The lamp is indicated at 48 in FIG. 3. I also position the lamp 48a substantial distance (i.e. 1 /2) away from the printing roll 22. At this distance it is necessary to employ a pair of reflectors 5! designed to concentrate the rays emitted by the lamp 4-8 at a central point designated at X on the surface of the wax transfer sheet overlying the copy and original on the roll 22. However, in order to prevent the reflectors 50 from absorbing too much heat and thereafter emitting long wave length radiation themselves, I provide them with an extremely bright specular gloss surface and make them as thin as possible. Also in order to get rid of air surrounding the lamp which has been heated, I provide a fan 52 directed at the space between the lower reflector 5% and the wax transfer sheet 15 causing a path of air to pass around the lamp 455 and outwardly through the spaces between it and the other ends of the reflectors 50. For the same purpose I do not extend the reflectors 59 around the lamp 43 because I find that the rays emitted from the back side of the lamp 48 tend generally to return to the lamp &8 and heat it excessively causing it to emit radiation in the lower and more absorbing end of the spectrum. In this way a substantial portion of the heat emitting from the lamp 48 is lost, but the concurrent advantage of concentrating the emitted spectrum in the higher and more penetrating end of the scale is gained.

The reflectors 50 concentrate the radiation from the lamp 4'8 primarily at a point X and in a direction generally normal to the tangent of the roll 22 at that point. As the light strikes the wax transfer sheet, it penetrates through it and through the copy sheet where it heats the surface of the letters on the original. These letters are indicated at 54 in FIG. 4. It will be understood, of course, that even the smallest letters are substantially wider than the sheets are thick. In the preferred embodiment of my invention the wax transfer and copy sheets are approximately .0015" in thickness and, as shown. in FIG. 4, the letter 54 being 10 times as wide as the sheets are thick, still represents about as small a thickness of letter as will be found in fine printing. As the light strikes the surface of the letter 54, it immediately heats the top thereof to relatively high temperature. I do not know what this actual temperature is, but I believe it is microscopically well above the boiling point of water. In any event, this surface immediately commences re-radiating energy backwardly through the copy to the transfer sheet, and the area A on the surface of the transfer sheet positioned above the letter 54 commences to heat up rapidly due to this re-radiation, while the area B overlying the background areas of the original heats up only to the extent that the wax transfer and copy sheets absorb the applied light from the lamp 48. In FIG. 5 I have shown a graph which illustrates the temperature rise in the areas A and B during the time of application of the light from the lamp 48. The temperature of the area A rises more rapidly and when it exceeds the melting point of the wax, the application of the radiation is terminated, this point being prior to the time at which the area B reaches the temperature of the melting point of the wax.

Since the heat returning from the surface of the letters 54 is at least in part radiant heat, it will be understood that ultimately close contact between the letters 54 and the copy sheet, or the copy sheet and the transfer sheet is not absolutely necessary. Of course, the heat radiates in all directions from the letters 54 and, therefore, spreading of the area A due to this cause will be minimized by close contact. No special precautions, however, have been taken for this purpose other than tensioning the transfer sheet 15 around the roll 22. Likewise letters, such as letter 54, are sometimes imprinted into the original as deeply as the thickness of the sheet, and naturally such imprinted letters cannot contact the copy sheet, but in view of the fact that the area A is heated at least in part by return radiation, such contact is not necessary.

As the combination of transfer sheet, copy sheet and original proceeds through the area of application of the heat and immediately following the point at which area A reaches the melting point of the wax, it then becomes highly important to insure a good contact between the wax transfer sheet and the copy sheet. This is done in the embodiment illustrated herein in FIG. 3 by means of a smooth, limp wiping surface 56 backed up by a sponge rubber pressure element 58. Of course, other forms of pressure mechanism, including soft rollers or accurately placed wiping vanes may suit the purpose. It is even feasible to secure good contact between the transfer and copy sheets by simply reducing the diameter of the roll 22 and advancing the point of application of the light X further around the roll so that the transfer sheet is pulled more tightly against the copy sheet.

The operation of my invention with the apparatus herein shown is extremely simple. The operator merely takes the original 44 with letters facing downwardly and places it in the opening 46. Thereafter the motor 30 and light source 24- are turned on simultaneously, and this in turn drives the wax transfer sheet 15 pulling the entire combination through the area X where the radiation is applied to the combination. As it passes through this area, the surface A of the wax transfer sheet melts and thereafter, passing under the pressure mechanism 56, transfers from the transfer sheet to the copy sheet. Immediately following this transfer, the roll 40 serves to separate the copy sheet from the transfer sheet and the copy and original issue through the opening 38 in the front of the machine. The copy is then severed from the unused portion thereof by operation of the knife 42. Since the copy carries on its surface the dyed wax in the areas of the letters, these letters are, to all intents and purposes, permanent. They are at least as permanent as most inks, and a good deal more permanent than pencil or carbon copy. The speed of throughput is extremely fast, and from start to finish it only takes about two seconds to make a copy. During repeated operation, the machine tends to gain heat and for this purpose the fan 52 is directed into the heated area causing cool air to flow through the reflector 50 and around the lamp 48. It is preferred, therefore, to have the fan 52 running at all times independently of the operation of the motor 30 and the light source 24. In regard to the cost of operation, it will be seen that the elements of the machine are all conventional and relatively inexpensive, the electrical power requirements are a relatively minor factor, and the copying materials are likewise inexpensive. While various forms of inexpensive paper will make copy by my machine to a more or less degree of satisfaction, I prefer a copy sheet 17 having a weight of approximately 16 pounds per ream and sold by the S. D. Warren Company under the trade name Monarco. 1 also can employ this same Monarco paper as the base for the wax transfer sheet 15, which 1 coat with an essentially paraflin based wax containing an aniline dye. Various forms of aniline dyes including blue, red and green have been employed. In fact, mixtures of these are satisfactory to produce black appearing letters, which curiously enough are not particularly absonbent to the applied radiation. To be sure, the copy of my invention will not reproduce further copies in my machine because the letters themselves are essentially transparent to the applied radiation.

It is to be understood, of course, that the smoothness of the surface of the copy sheet and transfer sheet is important in order to provide good wax transfer from the transfer sheet to the copy, but smoothness is also important for a less obvious reason. Since the return radiation from the letters is readily absorbed by the copy, certain portions of the copy immediately adjacent to the letters 54 reach a temperature higher than that of the wax transfer sheet surface in the area A. This undoubtedly acccounts for some lateral spreading due to conduction. In addition, the heated air in and adjacent to the copy is also capable of spreading the heat, and if the interface between the copy and the wax is rough, then the heated air will be squeezed laterally along the copy when the transfer pressure is applied thereto. This lateral spreading of the heated air is harmful because it melts the wax laterally and blurs the letters. In order to minimize this spreading of heated air, I preferably employ a relatively smooth upper surface for the copy sheet. In some cases I have noticed a definite improvement if the copy sheet contains a clear wax coating. A smooth mineral coated copy sheet has also been employed with good results. To date the best copies made by the process of my invention as regards sharpness of definition have been made employing a clear smooth thin cellulose acetate film as a copy sheet.

While it is possible to make excellent copies with my invention without providing for quick separation of the wax transfer sheet from the copy sheet, it will be understood that doing so requires a critical adjustment of the process variables in order to reduce the harmful effects of lateral conduction of heat surrounding the letters 54 not only in the original, but in the copy sheet and transfer sheet as well. Furthermore, since the copy sheet is in position to absorb a greater portion of the highly absorbent return radiation, the theoretical optimum performance under my invention cannot be attained without quick separation of the wax transfer sheet from the copy sheet as soon as the wax in the area A has been melted and transferred. In practice, however, it will be understood that the theoretical optimum used need not be attained, and since the residual heat in the copy sheet is at a temperature relatively close to the wax melting point good copy can be made without quick separation if the other variables are carefully regulated.

One of the advantages of quick separation is that it eliminates several variables and gives me a workable process without requiring extreme precision in the amount of heat-light applied. Thus with quick separation the amount of radiation applied to the sheets can vary anywhere from just enough radiation to substantially in excess of that required, and as long as the wax transfer sheet and copy sheet are separated rapidly the harmful effects of lateral conduction of the residual heat built up in the areas of the excess can be avoided. This simplifies construction and assembly by eliminating the need for extreme acuracy in the position of the lamp 48 or the contour of the reflectors 50.

Various ways of applying the radiation to the combined papers for carrying out my invention are feasible. For instance, it may be possible to direct a measured blast of radiation through a glass against which the combined sheets are held under pneumatic pressure. I have not been successful with such a mechanism in producing anything besides extremely poor fuzzy letters. I believe that my lack of success in this regard has come from the following factors: (a) the loss of heat from the wax in the area A into the glass by conduction due to the extremely close contact of the wax with the glass, (b) the lack of intensity of applied light reaching the letters, (0) the lack of any convenient way or mechanism for insuring quick separation of the wax from the copy sheet at a critical point of time in the process, and (d) the lack of a convenient way for applying pressure to transfer the wax from the transfer sheet to the copy sheet at the critical moment. Moreover, it is diflicult with a process employing a single blast of light to avoid applying more light to one portion than to another. However, such difficulties are purely mechanical and may be overcome once the importance of the various conditions of my process is known and understood. For this reason, I wish to make it clear that I do not intend to confine the scope of my invention to the precise form herein shown, but rather to measure it in terms of the appended claims which are directed severally to the precise form and to the invention in its broader aspects.

Again on the subject of alternate Ways for carrying out my invention, one drawback of the preferred embodiment of the apparatus herein shown is that it cannot make a copy from a page of a book because the original must follow around the roll 22.. This problem can be met by a device arranged to hold the page flat under vacuum and then passing the light source 24 over the page. I have done this with the light source and reflectors mounted on a moving unit which traverses the page at a measured rate during exposure. Quick separation can be arranged on such a unit by mounting a take-up reel for the used transfer sheet on the moving unit.

Since the best results with my invention are (short of harming the original) attained when the light intensity is highest, it should be particularly noted that I take pains not to interpose anything between the lamp 48 and the transfer sheet 15. Thus by applying the light directly to the back of the wax transfer sheet I am able to employ the full intensity of the radiation and in this way increase the available contrast between the return radiation from the letters and whatever radiation returns from the background areas on the surface of the original. I consider this to be an important. feature of my invention not hitherto employed, and to a large extent accounts for the success of my invention. It will be seen, furthermore, that employing the wax transfer sheet as a belt simultaneously performs the function of smoothing the wax paper and holding it tightly against the copy and the original. However, since the wax transfer sheet is under tension, it must not be stretched and the tension must be maintained relatively even or else random relative motion between the transfer sheet and the copy sheet will result in unwanted transfer of background across the entire sheet.

With the arrangement of my invention, it will be understood that control of the various important variables: in the process is relatively convenient. The motor 30 is provided with a variable speed mechanism. Moreover, the power source for the lamp 48 is provided with a rheostat in series so that minor adjustments in the intensity of the light can be made. In operation I find that the air surrounding the lamp 48 and the reflectors 50 tends to absorb heat, and as the machine stands idle this air as Well as the lamp and reflectors themselves tend to radiate heat into the area X of the printing roll 22. Generally speaking the fan 52 is adequate to convey such unwanted heat away from the area X. Also it helps to point the reflectors 50 in a general downward direction because otherwise it may be dificu-lt for the fan 52 actually to keep the hot air produced by the lamp 48 from coming in contact with the area A. In any event if such residual heat becomes troublesome, a simple expedient is to introduce a smooth backing sheet under the original.

In the application of the principles of my invention for the production of special forms of copy, it is entirely feasible to make the roll 22 quite large so that heavy bristol board prints and the like may be run through the machine without having to take an excessive bend. A large printing roll has the disadvantage of reducing the per square inch pressure and hence reduces the smoothing effect due to the tension of the wax sheet over the copy sheet. One important development of my invention relates to the production of prints similar to half-tone prints from a photographic gray scale. With my invention it is possible to place a fine mesh fabric between the origin-a1 and the copy and thereby print copies of photographs which have a general appearance similar to a half-tone. My success in producing such copies from gray scales is primarily due to the dependence of the process of my invention upon return radiation from the original.

While it is clear that my process makes use of return radiation, one must not consider that conduction and even convection are not also present, or that the mechanism of the return radiation does not employ conduction and convection to some extent. It will be understood that the copy sheet is relatively opaque to the long wave length return radiation. Thus the copy sheet will be heated by that portion of the return radiation and in turn re-radiate the heat onto the wax transfer sheet. In the process, conduction and even convection will be contributing to some extent to the progress of the heat through the copy. Of course, the specific heat of the copy sheet and also the lateral spreading of heat in the copy sheet will tend to reduce the intensity of such relayed return radiation, but since the letters are actually many time Wider than the paper is thick, the attenuation due to spreading is not too important. The main thing is that the heat, in Whatever form it takes, is capable of jumping two highly insulative air gaps and passing through an essentially nonconductive sheet of paper, on its way to melting the wax, and I intend to claim broadly the process herein described which brings this about.

While a basic object of my invention is the production of clear and precise copy, a by-product of the employment of re-radiation is the production of a negative effect in over exposed areas. With my process, when too much initial radiation has been applied for too long, the wax surrounding the letters melts and transfers to the copy. This might be expected to result in a general blurring of the letters, but with my process the wax immediately over the letters seems to thin out leaving the letters lighter than the thicker accumulation of transferred Wax adjacent thereto. I am not certain of the explanation of this negative effect, but I believe it is due to the fact that the wax is completely melted and of low viscosity in the return radiation area and spreads laterally by some form of surface tension to the areas adjacent to the letters where the viscosity of the wax (although melted) is higher, as the copy sheet and transfer sheet are separated. In any event, whatever causes the negative effect, it is collaterally important in that it ensures a degree of legibility in inadvertently over-exposed copy, and thereby results in a substantial saving.

It will be understood that the graph shown in FIG. 5 is only illustrative, and is used primarily as a means of quick explanation of the problem. A more careful analysis, however, of the temperature sequences of the various components of the problem should be considered. As the applied radiation strikes the wax transfer sheet and penetrates into the copy and the original, the entering radiation is absorbed relatively uniformly throughout the wax transfer sheet and the copy. However, the white areas of the original reflect some of the applied radiation back towards the transfer sheet While the black area do not. Thus the B areas on the wax transfer sheet for a short while actually start heating up slightly faster than the A areas. As the applied radiation continues, however, the surfaces of the letters commence heating up and re-radiating; but for a short additional period this reradiation is predominantly of such long wave length that little, if any, of it may be regarded as penetrating the copy and reaching the area A. The surface temperature of the letters, however, soon increases so rapidly that the return emission from the letters is of suflficient intensity and of short enough wave length to penetrate the copy and contribute significantly to melting the wax. If the applied radiation is then continued various things happen. Thus the surface temperature of the letters can rise too high and damage the original. The letters and heated portions of the copy can store too much residual heat, thus causing heat spreading of the letters unless quick separation precautions are taken. Also conduction of heat through the copy can take place, and since it will generally vary in relation to letter size, such conduction will tend to render the copy non-uniform. Accordingly it will be seen that the optimum results with my invention will be attained if the applied radiation is permitted to build up substantial superficial heat in the letters enough to make the return radiation a significant factor, but that it be terminated prior to the point at which the letters or the adjacent areas of the copy absorb too much residual heat. This analysis shows that the melting point of the wax must not be too low because, if it is relatively low, then the wax will melt due to applied radiation before the letters have had a chance to build up enough heat to start re-radiating with enough intensity and for suflicient time to contribute significantly to the melting of the wax in the A areas. I have found, for instance, that a wax with a melting point of about F. is much too fast. Copy can be made with it but a great deal of background appears. Generally speaking, I prefer a melting point of about F. The upper limit for wax melting point is influenced by several factors. Harm to the original may result if the throughput speed is too slow, also excessive spreading due to residual heat becomes a problem.

In addition, I also find it important to limit the thickness of the applied layer of wax on the transfer sheet. Wax is approximately twice as conductive as paper, and therefore, heat in the area A tends to spread through the wax resulting in blurred letters. I have found that excellent results are obtained when the transfer sheet with wax thereon is less than 1 mil in thickness. My best results have been obtained in using an all rag paper base sold by the S. D. Warren Company under the name Trent and a very thin wax coating. In this way, particularly with a relatively high melting point wax, and with intense heat emanating from the letters, the rate of temperature rise in the area of the letters greatly exceeds the heat dissipation rate in the wax. This means that sharp letters can be obtained by accurate termination of the applied radiation and/ or quick separation of the copy sheet from the transfer sheet.

It appears that papers which avoid scattering the radiation are particularly important. As the applied radiation passes through the wax transfer and copy sheets, it should go through by the most direct route and scattering at that point only needlessly or harmfully heats them up. Of course, when the heat returns from the letters through the copy to the wax, scattering is to be avoided to preserve definition. However, there is some advantage in having the transfer sheet scatter some of the radiation because the return radiation should not penetrate the transfer sheet too easily, and as long as the letters attain a sufficiently high surface temperature to transfer heat back to the wax surface, the fact that some scattering of the applied radiation takes place is not critical.

Having thus described and disclosed a preferred embodiment of my invention, what I claim as new and de sire to secure by Letters Patent of the United States is:

I. A process for copying radiant energy absorbing indicia on an indicia bearing sheet comprising: placing said indicia bearing sheet on a curved surface; placing a copy sheet over said indicia; placing a transfer sheet having a thermoplastic coating thereon over said copy sheet; drawing said transfer sheet into intimate contact with said copy sheet and pressing the same against said indicia bearing sheet by maintaining said transfer sheet under tension; both heating said transfer and copy sheets and causing heat from said indicia to cross the gap between said indicia and said copy sheet, to pass through said copy sheet, to cross the gap between said copy sheet and said transfer sheet and additionally to heat up said transfer sheet by directing incident radiation through said transfer and copy sheets to the surface of said indicia; and continuing said incident radiation only until the combined effect of it and said heat from said indicia melts said coating on said transfer sheet, whereby said coating transfers from said transfer sheet to said copy sheet in areas corresponding only to said indicia.

2. The copying process defined in claim 1 further characterized by continuously passing said transfer, copy and indicia bearing sheets past a radiation application zone; and shortly thereafter continuously separating the transfer sheet from the copy sheet after passage through said radiation application zone.

3. The process defined in claim 1 further characterized by continuously passing said transfer, copy and indicia bearing sheets past a radiation application zone, applying additional pressure to the exposed back of said transfer sheet closely following said radiation application zone and continuously separating said transfer sheet from said copy sheet shortly after the point of application of said additional pressure.

4. A process for copying radiant energy absorbing indicia on an indicia bearing sheet comprising; placing said indicia bearing sheet on a curved surface; placing a copy sheet over said indicia; placing a transfer sheet having a thermoplastic coating thereon over said copy sheet; drawing said transfer sheet into intimate contact with said copy sheet and pressing the same against said indicia bearing sheet by maintaining said transfer sheet under tension; both heating said transfer and copy sheets and causing heat from said indicia to cross the gap between said .indicia and said copy sheet, to pass through said copy sheet, to cross the gap between said copy sheet and said transfer sheet and additionally to heat up said transfer sheet by directing incident radiation against the exposed back of said transfer sheet and through said transi2 fer and copy sheets to the surface of said indicia; and continuing said incident radiation only until the combined effect of it and said heat from said indicia melts said coating on said transfer sheet, whereby said coating transfers from said transfer sheet to said copy sheet in areas corresponding only to said indicia.

5. The copying process defined in claim 4 further characterized by continuously passing said transfer, copy and indicia bearing sheets past a radiation application zone; and shortly thereafter continuously separating the transfer sheet from the copy sheet after passage through said radiation application zone.

6. The process defined in claim 4 further characterized by continuously passing said transfer, copy and indicia bearing sheets past a radiation application zone, applying additional pressure to the exposed back of said transfer sheet closely following said radiation application zone and continuously separating said transfer sheet from said copy sheet shortly after the point of application of said additional pressure.

7. A process for copying radiant energy absorbing indicia on an indicia bearing sheet comprising: placing said indicia bearing sheet on a smooth surface; placing a copy sheet over said indicia; placing a transfer sheet having a thermoplastic coating thereon over said copy sheet; continuously passing said so positioned sheets through a radiation application zone to heat said sheets and selectively melt said coating and cause it to transfer said copy sheet in areas corresponding to said indicia; and thereafter continuously separating said transfer sheet from said copy sheet at a given point shortly beyond said radiation zone by drawing said transfer and copy sheets in divergent directions.

References Cited in the file of this patent UNITED STATES PATENTS 2,503,758 Murray Apr. 11, 1950 2,769,391 Roshkind Nov. 6, 1956 2,808,777 Roshkind Oct. 8, 1957 2,844,733 Miller et al. July 22-, 1958 2,880,110 Miller Mar. 31, 1959 2,891,165 Kuhrmeyer June 16, 1959 2,897,090 Van Dam July 28, 1959 2,916,622 Nieset Dec. 8, 1959 

